Interface advance control in secondary recovery program by reshaping of the interface between driving and driven fluids and by retarding cusp formation

ABSTRACT

The interface between driving and driven fluids in a secondary recovery operation is reshaped after a cusp has developed by injection of fluids via control wells to delay the arrival of the injected driving fluid into the vicinity of a production well, and by retarding cusp formation via wells controlling the advance of flow gradients to spread the interface.

United States Patent Inventor Donald L. Hoyt Houston, Tex.

Appl. No. 837,783

Filed June 30, 1969 Patented Sept. 7, 1971 Assignee Texaco Inc.

New York, N.Y.

Continuation-impart of application Ser. No. 786,565, Dec. 24, 1968.

INTERFACE ADVANCE CONTROL IN SECONDARY RECOVERY PROGRAM BY RESHAPING OF THE INTERFACE BETWEEN DRIVING AND DRIVEN FLUIDS AND BY RETARDING CUSP FORMATION 14 Claims, 16 Drawing Figs.

US. Cl 166/245, 166/263, 166/268 Int. Cl E211! 43/22 Primary Examiner-1an A. Calvert Attorneys-K. E. Kavanagh and Thomas H. Whaley ABSTRACT: The interface between driving and driven fluids in a secondary recovery operation is reshaped after a cusp has developed by injection of fluids via control wells to delay the arrival of the injected driving fluid into the vicinity of a production well, and by retarding cusp formation via wells controlling the advance of flow gradients to spread the interface.

PATENTED SEP new 3.603, 395

v sum 3 or 3 INTERFACE ADVANCE CONTROL IN SECONDARY RECOVERY PROGRAM BY RESI'IAPING OF THE INTERFACE BETWEEN DRIVING AND DRIVEN FLUIDS AND BY RETARDING CUSP FORMATION CROSS-REFERENCES This application is a continuation-in-part application for patent of the copending, commonly assigned application for Pat. Ser. No. 786,565, filed Dec. 24, 1968 by Donald Lawrence Hoyt for Interface Advance Control in Secondary Recovery Program By Reshaping of The Interface Between Driving and Driven Fluids.

FIELD OF THE INVENTION This invention relates generally to the production of hydrocarbons from underground hydrocarbon-bearing formations, and more particularly, to a method for increasing the efficiency of the production of hydrocarbons therefrom by controlling the advance of the interface in pattern floods.

DESCRIPTION OF THE PRIOR ART In the production of hydrocarbons from permeable underground hydrocarbon-bearing formations, it is customary to drill one or more wells into the hydrocarbon-bearing formation and produce hydrocarbons, such as oil, through designated production wells, either by the natural formation pressure or by pumping the wells. Sooner or later, the flow of hydrocarbons diminishes and/or ceases, even though substantial quantities of hydrocarbons are still present in the underground formations.

Thus, secondary recovery programs are now an essential part of the overall planning for virtually every oil-and gas-condensate reservoir in underground hydrocarbon-bearing formations. In general, this involves injecting an extraneous fluid, such as water or gas, into the reservoir zone to drive formation fluids including hydrocarbons toward production wells by the process frequently referred to as flooding. Usually, this flooding is accomplished by injecting through wells drilled in a geometric pattern, the most common pattern being the fivespot.

When the driving fluid from the injection well reaches the production wells of a five-spot pattern, the areal sweep is about 71 percent. By continuing production considerably past breakthrough, it is possible to produce much of the remaining unswept portion. It would be great economic benefit to be able to achieve a sweep of 100 percent of the hydrocarbonbearing formation. It would be an even greater benefit to be able to achieve it at breakthrough, so that it would not be necessary to produce large quantities of injected driving fluid.

It is understood that the failure of the driving flood in secondary recovery operations to contact or sweep all the hydrocarbon area is due to the development of a cusp at the interface between the driving and the driven fluids which advances toward the production well. If other portions of the interface could be made to keep up, or if the cusp formation were delayed, a more complete areal sweep would be possible.

In the commonly assigned US. Pat. No. 3,393,735, issued to A. F. Altamira et al. on July 23, 1968 for Interface Advance Control in Pattern Floods by Use of Control Wells, there is disclosed how an increased amount of hydrocarbons is produced and recovered from an underground hydrocarbonbearing formation by employing at least three wells, penetrating such a formation, which wells are in line, to produce hydrocarbons from the formation via two of these wells including the middle well, as disclosed in the commonly assigned U.S. Pat. No. 3,109,487, issued to Donald L. Hoyt on Nov. 5, 1963 for Petroleum Production by Secondary Recovery. In both these cited patents, there is disclosed how a production control well is positioned between the injection well and the production well and is kept on production after the injected fluid reaches it. In this manner, the cusp is pinned down at the control well and while the area swept out by the injection fluid before breakthrough at the outer production well is increased, there is an unwanted handling of considerable quantities of injected fluid at the control well.

Another aspect to increase the sweep is disclosed in the commonly assigned US. Pat. No. 3,393,734, issued to D. L. Hoyt et al. on July 23, I968 and involves the retardation of the development of the cusp toward a production well. The method of achieving more uniform advance is to control the flow gradients so that the interface is spread" out. This can be done either by choosing a particular geometry of well positions or by adjusting the relative production rates so that the velocity of advance is not predominantly in one direction. It can be done also by shifting the gradients frequently, in both direction and magnitude, thus preventing any one section of an interface from advancing too far out of line.

SUMMARY OF INVENTION It is an overall object of the present invention to provide an improved secondary recovery procedure involving initially at least two production wells substantially in line with a source of driving fluid (such as an injection well or active aquifer), and a fourth well offset from such a line, which wells may be part of a well arrangement for exploiting a hydrocarbon-bearing formation, by reshaping the interface between a driving fluid and formation fluids, after development of a cusp, into a configuration favorable to greater sweep.

Such a four-well grouping is arranged with three wells substantially in line and a fourth well offset therefrom so that an end well in line is completed for injection and the remaining three wells are completed for production. Flooding is initiated at the end well by injection of an extraneous driving fluid, such as water or gas, thereinto and proceeds until breakthrough of the flood front occurs at either the closer of the production wells in line or the production well offset therefrom, at which time injection via the end well to maintain flooding is suspended, the production wells may be put on a standby basis and a small volume of an extraneous fluid is injected into the formation via the production well at which breakthrough occurred, to drive the cusp of the flood front therefrom and reshape the interface for the next production phase. Reshaping of the interface can be started at a predetermined time prior to breakthrough also. Then, injection is resumed at the end well and production is continued or resumed at the other production wells to spread the interface, and the production well into which the extraneous fluid was injected is shut in.

The interface has now been reshaped so that instead of just one point, viz, the tip of the cusp, being closest to the outer production well and thereby being accelerated to early breakthrough by the radial flow gradients, all the points over an appreciable portion of the interface will now be at approximately equal travel time away from the outer production well, this travel time being greater than would have been the time for breakthrough without injection of the extraneous fluid via the well suffering breakthrough. In addition, the flanks of the flood front, having been spread by production from the offset well, are reshaped by advancing during the period of injection of the extraneous fluid. Thus, more of the less accessible areas are swept, and the reversal of the cusp allows more sweep before breakthrough of the driving fluid into the outer production well. Examples of the extraneous fluid include produced formation hydrocarbon fluids, which may be treated with thickeners to increase the viscosity thereof, butane and propane, all being miscible with the formation fluids.

Other objects, advantages and features of this invention will become apparent from a consideration of the specification with reference to the figures of the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. I discloses four units of an inverted five-spot pattern (Prior Art);

FIG. la is illustrative of the interface advance in the form of a cusp toward a comer production well in one quadrant of such a five-spot pattern undergoing secondary recovery (Prior Art);

FIG. 2 illustrates the movement of the interface during phases of a production program in one quadrant of a l 3-well quadrilateral side well pattern undergoing secondary recovery (Prior Art HO. 3 discloses one quadrant of a nine-well diagonal pattern, illustrating the breakthrough at a comer production well, P following the reshaping of the cusp at the control well between the injection well and the comer production well resulting from injection of a small volume of production hydrocarbon fluids into a control well, with all other wells temporarily shut in;

FIGS. 40 and 4b and FIGS. 50 and 5b illustrate the effects of an offset production well in a five-well grouping on a line drive of at least three wells wherein reshaping of the cusp is undertaken at the in line production well suffering breakthrough;

FIGS. 60 and 6b illustrate an alternate showing of the effect of an offset production well with reshaping of the cusp at breakthrough thereat and the in line production well suffering breakthrough;

FIGS. 7a, 7b, 7c and 7d illustrate the application of the principle of FIG. 6a to uniformly spaced nine-well grouping, (including a seven-well geometric grouping), starting at one end of a diagonal axis thereof showing the effect of reshaping a spread interface; and

FIGS. 8a and 8b illustrate the progress of a flood front with a spread interface during phases of production in accordance with the disclosed method as applied in a l3-well grouping undergoing secondary recovery.

The objects of the invention are achieved by use of control wells to modify and reshape the interface after a cusp has developed thus delaying arrival of injected driving fluid into the vicinity of an outer production well, thereby obtaining through of the driving fluid into that well.

The specification and the figures of the drawings schematically disclose and illustrate the practice and the advantages of the invention with well patterns and areal sweep examples which are obtainable and have been observed both in secondary recovery operations and in potentiometric model studies which simulate secondary recovery operations. The model studies indicate a sweep obtained in an ideal reservoir, although the recovery from an actual sweep of a particular field may be greater or less, depending on field parameters.

Throughout the figures of the drawings, the same symbols will be maintained as follows: P P, and P, represent, respectively, production wells at the comers, along the sides, and the interior control wells of a pattern or well arrangement; and, a solid circle indicates a production well, a crossed circle indicates a shut-in well, an open circle a well site, and an arrowed open circle indicates an injection well. The diagonals xx in FIGS. 7a to 7d inclusive and 8a and 81; represents an axis of an injection well and a pair of in line production we.ls.

Referring to FIG. 1, there is disclosed four units of an inverted five-spot pattern unit are production wells, while the inner central well is used for injection.

FIG. la illustrates the growth of the cusp in one quadrant Jr an inverted five-spot pattern unit, wherein the secondary flooding fluid is injected into the central well and production is maintained at the comer wells until breakthrough, to result in a sweep of approximately 71 percent.

FIG. 2 illustrates the development of the cusp toward the comer production well, retarded by spreading of the gradients, production being maintained at both the comer (P and side production wells (P,) until breakthrough at the side wells (P,) for the end of the first phase, and then continuing production at the corner wells (P only, until breakthrough, for the end of the second phase.

Referring to rid; 3', there is disclosed aiequsarahi of a nine-well diagonal pattern, essentially the five-spot pattern with control wells positioned on the diagonals between the central injection well and the comer production wells. The control wells should be spaced at least one-half the distance between the central injection well and each corner production well, with the best results obtained when such control wells are positioned between three-quarters and seven-eighths of the distance from the central injection well toward the corner production wells. With such a pattern, the invention disclosed in the cited patent to Hoyt can be employed with success to increase the sweep area over that mentioned for the basic fivespot pattern.

Use of control wells as disclosed by above-cited patents to Hoyt and to Altamira et al., by continuing production from P, after breakthrough even to percent injected fluid, will have the beneficial effect but at the cost and difficulty of handling and disposing of large quantities of the driving fluid.

Instead, if a small volume, e.g. about IS percent of quadrant of a pattern unit volume of produced formation hydrocarbon fluids is injected into the formation via the control well, P from which production has ceased, the cusp will be driven back, and flank portions of the interface or flood front will be advanced (as indicated by the line A,--A, in FIG. 3), by the injected bubble of hydrocarbon fluids, shown in section by outline D The injected volume may be a percentage of the pore volume within the drainage radius of the well. A simple approximation of the drainage radius would be the average of the half distance to the nearest wells.

When production is initiated at the corner well, P with P shut in, and injection resumed at the central well, the interface and bubble both move toward the production well, P Now, however, all the points between a and b on the interface are approximately the same travel time from PC, and as the flood front progress, the bubble is reproduced and the points between a-b converge into the production well, yielding at breakthrough, a sweep of 87 percent, as outlined at B P B The advantage of high sweep is thereby realized without handling injection fluid.

If satisfactory production of reservoir fluid along with injection fluid is possible in a given reservoir, (it often is not), this percentage can be increased by continued production.

The improvement in sweep can be optimized in several ways. The size of the bubble, the geometry of the wells, the relative rate distribution and judicious use of the central injec tion well during the reshaping phase can be used in any given operation to mold the interface into the best shape for the next production phase.

FIG. 4a illustrates two phases of an in line production drive wherein the cusp interface at breakthrough at a production well 2 is reshaped by injection of a slug of produced hydrocarbon fluids into an underground formation via this well, as in- I dicated by the left diagonal lines, the dash outline indicating the end of the first phase, the second phase being the injection via well 2 and the situation as illustrated is the start of the third phase with injection via well 1, production via in line well 3 and offset well 4 to spread the cusp and well 2 being shut in, the sweep being indicated by right diagonals, showing spreading of the cusp toward well 4.

The dash outline in FIG. 4b is the sweep at breakthrough at well 4 at the end of the third phase, at which time this latter well is shut in and production is continued at well 3 till breakthrough thereat for the end of the fourth phase, leaving a lens of produced hydrocarbon fluids.

In FIG. 5a, production continues via wells 2 and 4 (and well 3 if so programmed) until breakthrough of the driving fluid injected via well 1, at which time the second phase is started with the breakthrough cusp being reshaped by injecting back into the formation a predetermined portion of produced hydrocarbon fluids via the well sufi'ering breakthrough, which I may be the production well in line, 2, as illustrated, or the offset well 4. Thus, the solid outline in FIG. 5a indicates the start of the third phase and also the spreading of the cusp due to production from the offset well 4.

The dash outline in FIG. 5b illustrates the sweep at the end of the third phase with breakthrough at the offset well 4, which is shut in then, and the fourth phase is completed with breakthrough at the end in line production well 3, leaving a lens of produced hydrocarbon fluids.

In FIG. 6a, there is illustrated the reshaping of the breakthrough cusps at wells 2 and 4, upon breakthrough of the driving fluid thereat, with injection via well I and production via well 3; and in FIG. 6b, the conclusion of production via well 3 is illustrated, injection being continued via well 1, wells 2 and 4 being shut in.

FIGS. 7a to 7d inclusive illustrate how the disclosed invention may be applied to a uniformly spaced nine-well grouping the production operation being initiated along a diagonal of wells 1, 2 and 3, continued from offset side wells 4 and 4', thence along the other diagonal through wells 6, 2 and 6', then the other offset wells 5 and 5 to conclusion at the remaining corner well 3.

Upon breakthrough of the driving fluid injected via well I at the side wells 4 and 4', as indicated in dash outline in FIG. 7a, the cusps are reshaped by injection of a predetermined slug of produced hydrocarbon fluids via these breakthrough wells, the remaining wells of the grouping being on a standby basis. Thereafter, as illustrated in FIG. 7b, injection via well 1 is resumed, side wells 4 and 4' are shut in and production is initiated at corner wells 6 and 6 and resumed at well 2, which in the previous phase had spread the interface and which will now suffer breakthrough. Thereafter, the cups interface is reshaped by injection of produced hydrocarbon fluids via well 2 and also via wells 6 and 6, which may have suffered breakthrough or will be about to do so and production initiated at wells 5, 5' and 3, at predetermined times. FIG. 70 illustrates the reshaping of the cusps upon breakthrough at side wells 5 and 5, which then are shut in and production resumed at the corner well 3 and injection at well 1. At breakthrough at well 3, only side slivers of the produced hydrocarbon fluids remain, the interface having been drawn in by well 3, as shown in FIG. 7d.

FIG. 8a discloses the basic nine-spot pattern modified by the addition of four interior control wells, P,, which can be positioned on the diagonals of the pattern for best advantage, as indicated previously. It can be visualized also as a four-unit, five-spot pattern wherein the injection wells of the inverted five-spot pattern units have been converted to production wells, and the innermost production well of the four-unit five spot pattern has been converted into an injection well. With such a conversion, the positions of the control wells have been predetermined and may not be situated for best effect.

As illustrated in one quadrant of the pattern, the first phase of the production method requires injecting driving fluid via the central well and production initiated and maintained at the interior and side wells of the pattern until breakthrough is achieved at the four interior wells, P as shown by the dash outline in FIG. 8a. Then these interior production wells are converted to injection wells for receiving produced formation hydrocarbon fluids (bubble D to reshape the cusp after which these wells are shut in and production is resumed from the side wells P, spreading the cusp until breakthrough thereat, as illustrated in FIG. 8a, the left diagonals in this and following figure indicating the returned hydrocarbon fluids. Then, as indicated in FIG. 8b, the four side wells are used to inject produced hydrocarbon fluids to reshape the breakthrough cusps and interface, as shown by dash outline, and then they are closed in, production is initiated and maintained at the corner production wells p until the interface has progressed through the stages indicated in FIG. 8b, viz. the bubble D being reproduced, the interface surface a-cb converges and breakthrough occurs. If production after breakthrough at the corner wells is continued, it becomes feasible to recover the teardrop bubbles for additional sweep.

In any kind of a secondary recovery operation in which one fluid is moved by another toward what is essentially a point sink, such as a production well, the radial flow gradients in the vicinity of the production well inevitably cause the interface to form a cusp pointing toward that well. The various fluid and field parameters, such as permeability distribution, viscosity ratio, well geometry, types of drives, miscibility, displacement efficiencies, etc., will cause the cusp to form earlier or later and be more or less pronounced, but it will form always, and always it will be detrimental to efficient sweep of any region being considered.

The ideal sweep situation for a reservoir would be one in which the fluid interface, as it approaches the last production well, somehow would be in such a geometrical position that all points on it would require the same time to reach the well. The method of spreading the cusp disclosed in US. Pat. No. 3,393,734 (to Hoyt et al.) is successful because it approaches this ideal.

There will be times, however, when the cusp cannot be spread, and the present disclosed combination method offers a convenient and workable alternative. All that is required is that there be at least two wells substantially in line with a source of driving fluid such as an injection well or an active aquifer, and a well offset from such a line to provide the spreading of the cusp interface. The principle involved is as follows:

The point of the cusp forms along the line of strongest gradient between the injection source and the nearest production well. If fluid is injected into the production well which is here called the control well, the system is reversed and the point of the cusp is driven back and away from the next production well in line, by the fluid injected via the control well. The interface between the control well fluid and the in' jection well fluid becomes concave. If the proper size of bubble is introduced, to fit the particular well geometry, the concavity may be sized such that all or most of the points on it (between a and b, e.g. in FIG. 3) will be in gradient fields such that their time of travel to the production well will be equal.

Resumption of injection of driving fluid and of production from the well beyond the control well will yield a very high sweep. The wider the distance between points a and b, the greater, in general, will be the sweep at breakthrough of these points into the producing well. This distance can be increased if the points of injection should straddle the axis extending through the injection and production wells. Alternatively, the use of production wells offset from the in line production wells will provide for the desired spreading of the cusp prior to use of such wells as injection wells.

Reservoir hydrocarbon fluids are suitable particularly here for control well injection fluid, since they will follow most closely model study predictions, and will be recovered without difficulty in the subsequent production phase. However, any fluid of properties (particularly viscosity and miscibility) similar to the reservoir hydrocarbon fluid-s can be used effectively.

Although emphasis has been placed in this disclosure on the practice of this invention as directed to a secondary recovery operation, particularly employing water or other similar aqueous fluid as the injection fluid or displacement fluid, as indicated hereinabove, the advantages obtainable in the practice of this invention are realized also in primary hydrocarbon production operations wherein the hydrocarbon bearing formation is under the influence of a water drive or gas drive, or both a water and gas drive, and also in the instance of a secondary recovery operation wherein a gas, such as natural gas, is employed as the injection fluid.

I claim:

I. A method of producing formation fluids including hydrocarbons from an underground hydrocarbon-bearing formation which comprises penetrating said formation with at least three wells substantially in line, a first well, a second well and a third well, the second and third wells being on one side of said first well with said second well closer thereto and a well offset from the line of wells and adjacent said second well, injecting an extraneous fluid into said formation via said first well to displace fluids including hydrocarbons in said formation toward said second and third wells and the offset well,

mation hydrocarbon fluids from produced formation fluids, ceasing injecting extraneous fluid and ceasing producing said formation fluids upon breakthrough of said extraneous fluid at a production well, thereupon injecting a portion of said formation hydrocarbon fluids into said formation via the production well suffering breakthrough and then shutting in the last mentioned well, thereafter resuming injecting extraneous fluid via said first well and producing formation fluids via said third well and the other original production well.

2. In a method as defined in claim 1, simultaneously producing said formation fluids including hydrocarbons from said formation via said second and offset wells.

3. In a method as defined in claim 1, converting said second and offset wells to injection wells for produced formation hydrocarbon fluids for a predetermined period as each suffers breakthrough of said extraneous fluid injected via said first well, thereafter shutting in each such well after said period has expired and producing formation fluids from said third well, said offset well being one of a pair which straddles the line of three wells.

4. In a method as defined in claim 1, said injecting of hydrocarbon fluids via said second well being of predetermined percentage of the pattern unit volume in the amount of about percent, to reshape the interface between said extraneous and formation fluids, said extraneous fluid being selected from the group consisting of butane, propane and produced hydrocarbon fluids.

5. In a method of producing formation fluids as defined in claim 1, said three wells in line being part of a five-well grouping, said offset well being one of a pair which straddles the line of said three wells.

6. In a method of producing formation fluids as defined in claim 1, said three wells in line being part of a seven well geometric grouping.

7. In a method of producing formation fluids as defined in claim 1, said three wells in line being part of a uniformly spaced nine-well grouping.

8. In a method as defined in claim 7, said second well being spaced away from said first well by at least three-quarters of the distance between said first well and said third well.

9. In a method of producing formation fluids as defined in claim 1, said three wellsin lin being part of a 13-well group- 7 ing, wherein the central well of said grouping is aninjection well and remaining grouping wells are production wells arranged equally along the sides and on the diagonals of a quadrilateral, said second and third wells being arranged along a diagonal thereof.

10. In a method of producing formation fluids as defined in claim 9, simultaneously initiating producing said formation fluids via all of said production wells.

11. A method of producing formation fluids including hydrocarbons from an underground hydrocarbon-bearing formation under the influence of an active aquifer which comprises penetrating said forrnation with a pair of production wells in line with the direction of advance of said aquifer, and a third production well offset from said line of production wells and adjacent one of said pair of production wells, producing formation fluids including hydrocarbons displaced by said aquifer via said production wells and said third production well until breakthrough of the interface between the formation fluids and said aquifer at a production well and thereupon ceasing producing formation fluids and then injecting via the production well suffering breakthrough an extraneous fluid of predetermined volume to reshape the interface between said extraneous and formation fluids where said breakthrough occurred, and thereafter shutting in this converted well, and then producing formation fluids including hydrocarbons from said formation via the remaining production wells.

12. In a method of producing formation fluids including hydrocarbons as defined in claim 11, said extraneous fluid being selected from the group consisting of butane, propane and roducedh drocarbon fluids.

l In a me od of producing formation fluids including hydrocarbons as defined in claim 12, said predetermined volume amounting to 15 percent of the reservoir volume within the drainage radius of the production well where said breakthrough occurred.

14. In a method of producing formation fluids includin hydrocarbons as defined in claim 11, said third production well being one of a pair of wells offset from the line of production wells. 

1. A method of producing formation fluids including hydrocarbons from an underground hydrocarbon-bearing formation which comprises penetrating said formation with at least three wells substantially in line, a first well, a second well and a third well, the second and third wells being on one side of said first well with said second well closer thereto and a well offset from the line of wells and adjacent said second well, injecting an extraneous fluid into said formation via said first well to displace fluids including hydrocarbons in said formation toward said second and third wells and the offset well, producing said formation fluids including hydrocarbons from said formation via the second and offset wells, recovering formation hydrocarbon fluids from produced formation fluids, ceasing injecting extraneous fluid and ceasing producing said formation fluids upon breakthrough of said extraneous fluid at a production well, thereupon injecting a portion of said formation hydrocarbon fluids into said formation via the production well suffering breakthrough and then shutting in the last mentioned well, thereafter resuming injecting extraneous fluid via said first well and producing formation fluids via said third well and the other original production well.
 2. In a method as defined in claim 1, simultaneously producing said formation fluids including hydrocarbons from said formation via said second and offset wells.
 3. In a method as defined in claim 1, converting said second and offset wells to injection wells for produced formation hydrocarbon fluids for a predetermined period as each suffers breakthrough of said extraneous fluid injected via said first well, thereafter shutting in each such well after said period has expired and producing formation fluids from said third well, said offset well being one of a pair which straddles the line of three wells.
 4. In a method as defined in claim 1, said injecting of hydrocarbon fluids via said second well being of predetermined percentage of the pattern unit volume in the amount of about 15 percent, to reshape the interface between said extraneous and formation fluids, said extraneous fluid being selected From the group consisting of butane, propane and produced hydrocarbon fluids.
 5. In a method of producing formation fluids as defined in claim 1, said three wells in line being part of a five-well grouping, said offset well being one of a pair which straddles the line of said three wells.
 6. In a method of producing formation fluids as defined in claim 1, said three wells in line being part of a seven well geometric grouping.
 7. In a method of producing formation fluids as defined in claim 1, said three wells in line being part of a uniformly spaced nine-well grouping.
 8. In a method as defined in claim 7, said second well being spaced away from said first well by at least three-quarters of the distance between said first well and said third well.
 9. In a method of producing formation fluids as defined in claim 1, said three wells in line being part of a 13-well grouping, wherein the central well of said grouping is an injection well and remaining grouping wells are production wells arranged equally along the sides and on the diagonals of a quadrilateral, said second and third wells being arranged along a diagonal thereof.
 10. In a method of producing formation fluids as defined in claim 9, simultaneously initiating producing said formation fluids via all of said production wells.
 11. A method of producing formation fluids including hydrocarbons from an underground hydrocarbon-bearing formation under the influence of an active aquifer which comprises penetrating said formation with a pair of production wells in line with the direction of advance of said aquifer, and a third production well offset from said line of production wells and adjacent one of said pair of production wells, producing formation fluids including hydrocarbons displaced by said aquifer via said production wells and said third production well until breakthrough of the interface between the formation fluids and said aquifer at a production well and thereupon ceasing producing formation fluids and then injecting via the production well suffering breakthrough an extraneous fluid of predetermined volume to reshape the interface between said extraneous and formation fluids where said breakthrough occurred, and thereafter shutting in this converted well, and then producing formation fluids including hydrocarbons from said formation via the remaining production wells.
 12. In a method of producing formation fluids including hydrocarbons as defined in claim 11, said extraneous fluid being selected from the group consisting of butane, propane and produced hydrocarbon fluids.
 13. In a method of producing formation fluids including hydrocarbons as defined in claim 12, said predetermined volume amounting to 15 percent of the reservoir volume within the drainage radius of the production well where said breakthrough occurred.
 14. In a method of producing formation fluids including hydrocarbons as defined in claim 11, said third production well being one of a pair of wells offset from the line of production wells. 